Giant optical absorption and ferroelectric polarization of BiCoO 2 S perovskite oxysulfide by first principles prediction

Obtaining an ideal ferroelectric photovoltaic (FE-PV) material with a narrow bandgap and a large ferroelectric polarization value can enable us to achieve great practical FE-PV performance. By the introduction of sulfur into the tetragonal BiCoO perovskite with a C-type antiferromagnetic ordering, it is found that the bandgap of BiCoO S decreases significantly (about 1.2 eV) while maintaining a large polarization value (about 1.86 C m ) that is similar to the value of 1.793 C m of BiCoO . Most noteworthy is that the optical absorption of BiCoO S is remarkably higher than those of BiCoO and other FE-PV materials. The decrease of the BiCoO S bandgap originates from the movement of Co 3d states to a low-energy position due to the reduction of the Co ionicity when the less electronegative sulfur is introduced into BiCoO to substitute oxygen. The narrow bandgap and the high optical absorption of the BiCoO S films grown on different substrates are favorable for FE-PV applications. In addition, the bandgap of BiCoO S can be modulated by the doping amount of sulfur, which can help us fabricate multilayer FE-PV devices based on different bandgaps from different layers.